It is known, particularly for network switches, to interconnect a multiplicity of network units into a switching ‘fabric’ so that in effect, and particularly in relation to the external network, the units act as a single network entity. One of the purposes of a fabric of network units is the provision of ‘resiliency’ that is to say the ability of the switching fabric to continue operation, so far as is feasible, notwithstanding a fault in or removal of one of the units of the fabric or one of the links connecting the units of the fabric. Early forms of such a fabric were in the form of a cascaded ring although more recently mesh type systems have been proposed. It has also been proposed, for example in the prior copending U.S. patent application to Weyman et al, Ser. No. 10/093,506 filed 11 Mar. 2002 (which shares inventors and a common assignee herewith) to provide distributed routing for unicast packets. The distributed routing described in that application, which is incorporated by reference herein, requires the action of routing, which necessarily involves changes in a MAC address, to be performed by the ‘ingress’ unit, that is to say the unit, within the fabric, which first receives the packet. If the egress port by which the packet leaves the fabric is on a different unit, then after the actions required by routing, the packet would be forwarded by bridging to the unit on which the egress port is located. In this way, although there may be multiple routers in the fabric, only one is involved in the routing of any single packet, there will be only one routing ‘hop’ and so it will appear to entities outside the fabric as if the fabric contained a single router.
However, the necessary change in the destination MAC address renders such a distributed unicast forwarding scheme unsuitable for routing multicast packets, for which the MAC address should not change within the switching fabric.
Accordingly, a primary object of the present invention is to provide a switching fabric, and units for use within such a switching fabric, which can provide distributed multicast routing.
A further object of the invention is to minimise the usage of bandwidth on links between the units of such a fabric. A yet further object of the invention is to provide consistency and compatibility with bridging fabrics and unicast routing fabrics.